Ministry of Higher Education and Scientific Research

Al-Mustaqbal University
College of Engineering & Technology
Medical Instrumentation Techniques Engineering Department
Electrical Technology
Third Class

Weeks 3 & 4

Testing in Transformers

By Osamah Jaber Ghayyib

1. Introduction

All the transformers are tested before placing them in the field. By performing these tests,
we can determine the parameters of a transformer to compute its performance characteristics (like
voltage regulation and efficiency etc.). Large transformers cannot be tested by direct loading
because of the following reasons:

(i) It is almost impossible to arrange such a large load required for direct loading.

(ii) While performing test by direct loading, there is huge power wastage.

(iii) It is very inconvenient to handle the power equipment.

1.1 Types of Transformer Tests

Tests of transformer done at the manufacturer’s premises –

• Type tests.
• Routine tests.
• Special tests.

Tests of transformer done at the consumer’s site

• Pre-commissioning tests.
• Periodic tests.
• Emergency tests.

1.1.1 Type Tests of Transformer

The type tests of a transformer are performed at the manufacturer’s premises to prove the
design expectations and consumer’s specifications. The type tests are performed in a prototype
unit, not in all manufactured units and these tests confirm the basic and main design criteria of
the transformer.

1
 Following transformer tests are included in the type tests –

• Transformation ratio test.

• Winding resistance test of transformer.
• Measurement of core losses and no-load current through open-circuit test.
• Measurement of short circuit impedance and copper losses through short-circuit
test.
• Transformer vector group test.
• Measurement of insulation resistance.
• Dielectric test of transformer.
• On-load tap-changer test.
• Temperature rise test.
• Vacuum test of tank and radiators.

1.1.2 Routine Tests of Transformer

The routine tests of a transformer are performed to confirm the operational performance of
the transformer and being performed on every unit manufactured.

Following tests are included in the routine tests –

• Winding resistance test of transformer

• Transformation ratio test
• Measurement of core losses and no-load current through open-circuit test
• Measurement of short circuit impedance and copper losses through short-circuit
test
• Transformer vector group test
• Dielectric test of transformer
• On-load tap-changer test
• Oil pressure to check against the leakages of joints and gaskets.

2
1.1.3 Special Tests of Transformer
The special tests of transformer are performed depending upon customer’s requirements to
gathering the information which is useful during the operation and maintenance of the
transformer.

The special tests of transformer include the following tests −

• Dielectric test of transformer

• Short-circuit test
• Measurement of acoustic noise level
• Measurement of no-load current harmonics
• Measurement of zero-sequence impedance of the 3-phase transformer
• Measurement of power drawn by cooling fans and oil pumps
• Tests on accessories e.g. buchholz relay, oil preservator, temperature indicators,
pressure relief devices etc.

To furnish the required information open circuit and short circuit tests are conducted
conveniently without actually loading the transformer. The other important tests which are
conducted on a transformer are polarity test voltage ratio test and Back-to-back test.

2. Polarity test

Polarity test is performed to determine the terminals with same instantaneous polarity of
the two windings when terminals are not being marked. The relative polarities of the primary and
secondary terminals are required to be known for

(i) interconnecting two or more transformers in parallel.

(ii) connecting three single-phase transformers while doing poly-phase transformation of

power.

(iii) connecting windings of the same transformer in parallel or series.

3
 For determining the relative polarity of the two windings of a transformer, the two winding
are connected in series and a voltmeter is connected across them as shown in Fig. 1. One of the
winding (preferably HV winding) is excited from a suitable AC voltage (less than rated value). If
the polarities of the windings are as marked on the diagram, then the windings will have a
subtractive polarity and the voltmeter will read the difference of E1 and E2 (i.e., E1 – E2). If the
voltmeter reads E1 + E2 the polarity marking of one of the windings must be reversed.

Fig 1. Block diagram for Polarity test.

3. Open circuit test

This test is carried out at rated voltage to determine the no-load loss or core loss or iron
loss. It is also used to determine no-load current 𝐼0 which is helpful in finding the no-load
parameters i.e., exciting resistance 𝑅0 and exciting reactance 𝑋0 of the transformer.

Usually, this test is performed on low voltage side of the transformer, i.e., all the measuring
instruments such as voltmeter (V), wattmeter (W) and ammeter (A) are connected in low-voltage
side (say primary). The primary winding is then connected to the normal rated voltage V1 and
frequency as given on the name plate of the transformer. The secondary side is kept open or
connected to a voltmeter V′ as shown in Fig. 2.

Since the secondary (high voltage winding) is open circuited, the current drawn by the
primary is called no-load current 𝐼0 measured by the ammeter A. The value of no-load current 𝐼0

4
is very small usually 2 to 10% of the rated full-load current. Thus, the copper loss in the primary
is negligibly small and no copper loss occurs in the secondary as it is open. Therefore, wattmeter
reading 𝑊0 only represents the core or iron losses for all practical purposes. These core losses are
constant at all loads. The voltmeter V′ if connected on the secondary side measures the secondary
induced voltage V2.

The ratio of voltmeter readings, V2/ V1 gives the transformation ratio of the transformer

Fig 2. Open circuit test

The Iron losses measured by this test are used to determine transformer efficiency and
parameters of exciting circuit of a transformer shown in Fig. 3

Fig 3. Equivalent circuit of transformer at open test.

5
 In table below, a set of equation can be derived by performing this test (if the test is
performed at the primary side).

Symbols (test on Symbols (test on

Parameter Description
primary) secondary)
Voltmeter reading 𝑉1 𝑉2
Ammeter reading 𝐼0 𝐼0′
Wattmeter reading or
iron losses
𝑊0 = 𝑉1 𝐼0 cos 𝜙0 𝑊0 = 𝑉2 𝐼0′ cos 𝜙0
𝑊0 𝑊0
Working component 𝐼𝑤 = 𝐼𝑤′ =
𝑉1 𝑉2
2
Magnetizing component 𝐼𝜇 = √𝐼0 2 − 𝐼𝑤 2 𝐼𝜇′ = √𝐼0′ − 𝐼𝑤′ 2

𝑉1 𝑉2
Exciting resistance 𝑅0 = 𝑅0′ =
𝐼𝑤 𝐼𝑤′
𝑉1 𝑉2
Exciting reactance 𝑋0 = 𝑋0′ =
𝐼𝜇 𝐼𝜇′

4. Short circuit test

This test is carried out to determine the following:

(i) Copper losses at full load (or at any desired load). These losses are required for the
calculations of efficiency of the transformer.

(ii) Equivalent impedance (Z01 or Z02), resistance (R01 or R02) and leakage reactance (X01
or X02) of the transformer referred to the winding in which the measuring instruments are
connected. Knowing equivalent resistance and reactance, the voltage drop in the transformer can
be calculated and hence regulation of transformer is determined.

6
 Fig 4. Short circuit test

This test is usually carried out on the high-voltage side of the transformer i.e., a wattmeter
W, voltmeter V and an ammeter A are connected in high-voltage* winding (say secondary). The
other winding (primary) is then short circuited by a thick strip or by connecting an ammeter A′
across the terminals as shown in Fig. 4. A low voltage at normal frequency is applied to the high
voltage winding with the help of on autotransformer so that full-load current flows in both the
windings, measured by ammeters A and A′. Low voltage is essential, failing which an excessive
current will flow in both the windings which may damage them.

𝑅02 𝑋02
𝐼2𝑠𝑐

𝑉2𝑠𝑐 𝑍02

Fig 5. Equivalent circuit of transformer at Short circuit test (Secondary side)).

The iron losses are negligibly small due to low value of flux as these losses are
approximately proportional to the square of the flux. Hence, wattmeter reading Wc only represents
the copper losses in the transformer windings for all practical purposes. The applied voltage V2sc

7
is measured by the voltmeter V which circulates the current I2sc (usually full load current) in the
impedance Z02 of the transformer to the side in which instruments are connected as shown in
Fig. 5.
In table below, a set of equation can be derived by performing this test.

Symbols (test on Symbols (test on

Parameter Description
secondary) primary)
Voltmeter reading 𝑉2𝑠𝑐 𝑉1𝑠𝑐
Ammeter reading (first ammeter) 𝐼2𝑠𝑐 𝐼1𝑠𝑐
Wattmeter reading 2
𝑊𝑐 = 𝐼2𝑠𝑐 𝑅02 2
𝑊𝑐 = 𝐼1𝑠𝑐 𝑅01
Equivalent resistance referred to 𝑊𝑐 𝑊𝑐
𝑅02 = 2 𝑅01 = 2
secondary 𝐼2𝑠𝑐 𝐼1𝑠𝑐
Equivalent impedance referred to 𝑉2𝑠𝑐 𝑉1𝑠𝑐
𝑍02 = 𝑍01 =
secondary 𝐼2𝑠𝑐 𝐼1𝑠𝑐
Equivalent reactance referred to
𝑋02 = √𝑍02 2 − 𝑅02 2 𝑋01 = √𝑍01 2 − 𝑅01 2
secondary

Examples 1: A 15 kVA, 440/230 V, 50 Hz, single phase transformer gave the following
test results:

Open Circuit (LV side) 250 V, 1.8A, 95 W.

Short Circuit Test (HV side) 80 V, 12.0 A, 380 W.

Compute the parameters of the equivalent circuit referred to LV side.

Solution

Transformer rating = 15 kVA; E1 = 440 V; E2 = 230 V; f = 50 Hz

Open circuit test (LV side); 𝑉2 = 250 V; 𝐼0′ = 1.8 A; 𝑊0 = 95 W
Short circuit test (HV side); 𝑉1𝑠𝑐 = 80 V; 𝐼1𝑠𝑐 = 12 A; 𝑊𝑐 = 380 W

8
 From open circuit test performed on LV side;
𝑊0 95
𝐼𝑤′ = = = 0.38 A
𝑉2 250

2
𝐼𝜇′ = √𝐼0′ − 𝐼𝑤′ 2 = √(1.8)2 − (0.38)2 = 1. 75943 A

𝑉2 250
𝑅0′ = = = 658 Ω
𝐼𝑤′ 0.38
𝑉2 250
𝑋0′ = ′
= = 142 Ω
𝐼𝜇 1. 75943

From short circuit test performed on HV side;

𝑉1𝑠𝑐 80
𝑍01 = = = 6.667 Ω
𝐼1𝑠𝑐 12

𝑊𝑐 380
𝑅01 = 2 = = 2.639 Ω
𝐼1𝑠𝑐 (12)2

𝑋01 = √𝑍01 2 − 𝑅01 2 = √(6.667 )2 − (2.639)2 = 6. 122 Ω

230
𝑘= = 0.5227
440

Transformer resistance and reactance referred to LV (secondary) side;

𝑅02 = 𝑅01 × 𝑘 2 = 2.639 × (0.5227)2 = 0.7211 Ω

𝑋02 = 𝑋01 × 𝑘 2 = 6. 122 × (0.5227)2 = 2.673 Ω

Examples 2: Open-circuit and short-circuit tests on a 4 kVA, 200/400 V, 50 Hz, one-

phase transformer gave the following test:

O.C. test: 200 V, 1 A, 100 W (on L.V. side)

9
S.C. test: 15 V, 10 A, 85 W (with primary short-circuited)

Draw the equivalent circuit referred to primary:

Solution

Transformer rating = 4 kVA; E1 = 200 V; E2 = 400 V

Open circuit test (LV side); 𝑉1 = 200 V; 𝐼0 = 1 A; 𝑊0 = 100 W
Short circuit test (HV side); 𝑉2𝑠𝑐 = 15 V; 𝐼2𝑠𝑐 = 10 A; 𝑊𝑐 = 85 W

From open circuit test performed on LV side;

𝑊0 100
𝐼𝑤 = = = 0.5 A
𝑉1 200

𝐼𝜇 = √𝐼0 2 − 𝐼𝑤 2 = √(1)2 − (0.5)2 = 0.866 A

𝑉1 200
𝑅0 = = = 400 Ω
𝐼𝑤 0.5
𝑉1 200
𝑋0 = = = 231 Ω
𝐼𝜇 0.866

From short circuit test performed on HV side;

𝑉2𝑠𝑐 15
𝑍02 = = = 1.5 Ω
𝐼2𝑠𝑐 10

𝑊𝑐 85
𝑅02 = 2 = = 0.85 Ω
𝐼2𝑠𝑐 (10)2

𝑋02 = √𝑍02 2 − 𝑅02 2 = √(1.5)2 − (0.85)2 = 1.236 Ω

400
𝑘= =2
200

Transformer resistance and reactance referred to primary side;

10
 𝑅02 0.85
𝑅01 = = = 0.21 Ω
𝑘2 (2)2

𝑋02 1.236
𝑋01 = = = 0.31 Ω
𝑘2 (2)2

The equivalent circuit referred to primary side is shown below

𝑅01 𝑋01

5. Back-to-back Test

The open-circuit test and short circuit test are performed to determine the equivalent circuit
parameter. With the help of these tests, we cannot find the temperature rise in a transformer.
Because the open-circuit test is examined only core loss and short-circuit test is examined only
copper loss. However, the transformer is not subjected concurrently to both losses. Hence, the
alternative is Sumpner’s test.

The solution to this problem is the Sumpner’s test. The Sumpner’s test is performed to
determine the transformer efficiency, voltage regulation, and heating effect of the transformer
under loading conditions. The Sumpner’s test is also known as the back-to-back test as this test
consists of two identical transformers connected back-to-back.

In Sumpner’s test, actual loading conditions are simulated without connecting actual load.
For a small transformer, it is convenient to connect full-load. But it is difficult to connect full-
load in the case of large transformers. Therefore, this test helps to find the important parameters

11
of the transformer. And the Sumpner’s test gives more accurate results compared to open-circuit
and short-circuit tests.

To perform the Sumpner’s test, two single-phase transformers with identical ratings are
required. The experimental circuit diagram of the Sumpner’s test is shown in the figure below.

Fig 6. Experimental circuit diagram of the Sumpner’s test.

12